This book constitutes the refereed proceedings of the 27th International Conference on Developments in Language Theory, DLT 2023, held in Umeå, Sweden, during June 12–16, 2023. The 20 full papers included in this book were carefully reviewed and selected from 32 submissions (31 regular ones and one invited).The DLT conference series provides a forum for presenting current developments informal languages and automata. Its scope is very general and includes, among others, the following topics and areas: grammars, acceptors and transducers for words; trees and graphs; relations between formal languages and artificial neural networks; algebraic theories of automata; algorithmic, combinatorial, and algebraic properties of words and languages; variable length codes; symbolic dynamics; cellular automata; groups and semigroups generated by automata; polyominoes and multidimensional patterns; decidability questions; image manipulation and compression; efficient text algorithms; relationships to cryptography, concurrency, complexity theory, and logic; bio-inspired computing; and quantum computing.